From the production of metal wires and more especially of wires from non-ferrous metals such as copper, aluminium or brass, an extrusion press is generally employed.
In the method known as direct extrusion a hot metal ingot or billet is placed inside a fixed container the end of which is equipped with a die of the required diameter. Through the other end of the container is introduced a rammer which compels the metal of the billet to pass through the die to form the wire.
In the method known as inverse extrusion the rammer consists of a fixed tubular column carrying the die at the end of it. The ingot is placed in a container against which bears a movable crosshead actuated by a jack which causes threading of the container over the rammer with formation of the wire.
In both cases one is restricted in the diameter of the wire obtained and it is difficult to get below a diameter of 7 to 8 mm, depending upon the power of the press.
In order to reduce the diameter of the wire further, it may be subjected to rolling by passing between a pair of rollers which roll practically one against the other, each bearing a groove of cross-section equal to half the cross-section of the wire which it is desired to obtain.
If the wire leaves the die at a velocity V.sub.1 with an area of cross-section S.sub.1 and the area of passage between the rollers is S.sub.2 the wire passes between the rollers at a velocity of transit V.sub.2 which is connected with V.sub.1 in a ratio which is the inverse of the ratio between the corresponding areas of cross-section. That is, one has: ##EQU1##
Consequently the rollers for reduction in area must be driven in rotation so that their peripheral velocity V.sub.3 in the groove is substantially equal to the velocity of transit V.sub.2 which has just been defined.
The Applicant Company has already proposed in the French Pat. No. 2.360.359 filed on Aug. 5, 1976, to subject the wire to drawing after leaving the die, this drawing being caused by rollers for reduction in area. With this object the latter must be driven at a peripheral velocity V.sub.3 higher than V.sub.2. In that way the area of the wire is reduced by a combined effect of traction and rolling. However, the velocity of leaving the die may vary and this is in particular the case when the extrusion of a number of wires is being carried out simultaneously from one and the same ingot by means of a number of dies side by side.
That is, it has already been observed that in the extrusion of brasses in two jets the leaving velocities of the products are generally different for numerous reasons the main ones of which are:
a geometric difference in origin and a different deformation of the dies, PA1 a different clogging of the two dies, PA1 and above all a heterogeneity of temperature of the billet in which the plane of symmetry of the distribution of the temperatures across a cross-section has no reason in general to coincide with the plane of symmetry perpendicular to the axis common to the two dies. This difference in velocity presents a disadvantage especially in the reeling of the wires on reelers located after the die, the speed of rotation of which must be regulated as a function of the velocity of transit of the wire. Hitherto it has been observed, however, that the various parameters of extrusion were stabilized at the time of an extrusion series and that the difference in velocity was substantially repetitive from one extrusion to the next. Hence a substantially constant correction may be made of the speeds of rotation of the reelers which receive the products. Thus in practice a correction of velocity may be carried out, of the order of more or less 15% about the mean velocity.
But when the area of cross-section is reduced between two rollers this difference in velocity may have another disadvantage.
That is, in the event of increase in the velocity V.sub.1 of leaving the die there is produced upstream of the rollers for reduction in area, an influx of material which may lead to the formation of fins by the passing of the excess metal between the rollers on opposite sides of the wire. Hence one would be obliged if one wanted always to produce a drawing of the wire, to subordinate the speed of rotation of the rollers to the velocity V.sub.1 of leaving the die.